The present invention relates to an actuator for a drive mode shifting device for an automobile and, in particular, to the actuator for shifting drive modes, for example, between a two-wheel drive mode and a four-wheel drive mode.
A conventional actuator for a driving mode shifting device is disclosed, for example, in Japanese Patent Laid-Open Publication No. H(Heisei) 8-223983. In this publication, the actuator for shifting drive modes between a two-wheel drive mode and a four-wheel drive mode is disclosed. The actuator includes a motor, a rod, a torque transmitting mechanism and a motor control mechanism. The motor outputs rotational torque according to an electric current. The rod can slide along the axis direction thereof so as to shift the drive modes. The torque transmitting mechanism changes the rotational torque to forwarding power (the propelling power), and transmits the forwarding power to the rod, when the motor is energized. The torque transmitting mechanism rotates the motor in the opposite direction by means of a reaction, which is received by the rod, when the motor is deenergized. The motor control mechanism fixes a predetermined period for providing electric current to the motor. When the motor has received more than the predetermined electric current, the motor control mechanism controls in the PWM (pulse width modulation) control so as to prevent the output torque from being more than a predetermined torque.
In this invention, the electric current to the motor is limited so as to control the output torque of the motor. Thus, the rod and the torque transmitting mechanism do not receive any great loads so that the torque transmitting mechanism and other parts become small.
However, in the above actuator, the predetermined period for providing electric current to the motor, is fixed. For example, the period is 1 second in the embodiment thereof. Therefore, even if the shifting drive mode between the two wheel drive mode and the four-wheel drive mode is not completely finished, the electric current is supplied for the predetermined period (1 second).
Further, in the above actuator, the two drive modes are shifted according to the positions of the rod. In detail, one end of the movable length of the rod is the two-wheel drive mode, and the other end of the movable length of the rod is the four-wheel drive mode. However, since the above actuator does not have a detecting means for detecting the position of the rod, the actuator is not acceptable for a complicated driving mode shifting. The complicated driving mode shifting device selects from three drive modes, for example, two-wheel drive mode, four-wheel drive mode (with the center differential system being locked) and four-wheel drive mode (with the center differential system being free).
In addition, in the above conventional actuator, the big rotational torque of the motor is essential to slide the rod, since the torque transmitting mechanism rotates the motor in the opposite direction by means of the reaction. Thus, for the purpose of a sure shifting of the drive mode, electric current to supply the motor is always controlled to make large torque. Accordingly, it is difficult to use a small motor so as not to prevent the motor from becoming overheated.
The invention has been conceived to solve the above-specified problems. According to the invention, there is provided an actuator for a driving mode shifting device comprising: a motor having a motor shaft which outputs rotational torque according to electric current; an output member, which is moved by the rotational torque, for shifting drive modes according to positions thereof; a torque transmitting mechanism for transmitting the rotational torque from the motor shaft to the output member; a control mechanism for selecting drive modes and for controlling the electric current to energize the motor so as to provide the selected drive mode; a one-way rotational mechanism disposed between the motor shaft and the torque transmitting mechanism for preventing the motor shaft rotating against the motor; a rotational torque absorption mechanism disposed between the one-way rotational mechanism and the torque transmitting mechanism for absorbing the rotational torque, when the motor shaft is rotated and the output member can not be moved; and a rotational angle detecting means for detecting the rotational angle of the motor shaft.
Other objects and advantages of invention will become apparent during the following discussion of the accompanying drawings.